(1) Field of the Invention
The present invention relates to solder shields as used in the volume production of printed circuit cards, and more particularly to a shield for protecting the gold tabs of these cards.
(2) Description of the Prior Art
At present Titanium strips are pressed over the gold plated tabs to prevent the tabs from being contacted by the hot solder during the wave soldering process. The strips are made of Titanium as that material resists the adherence of solder. However, microscopic dots of solder do adhere to the tab protectors.
During handling of the tab protectors after removal from the cards, microscopic pieces of solder become dislodged and work their way into the tab protector slot. When the tab protector is placed on subsequent cards and the card passes over the molten solder wave, the tab protector heats up and the microscopic bits of solder on the inside of the protector melt and transfer to the gold plated tabs on the cards. Because of the excellent solderability of the gold surface the molten solder spreads on the gold surface resulting in solder dots up to 0.005 inches in diameter on the tabs. This cannot be allowed as the connector that the card plugs into has only a small gold dot that mates with the gold tabs and if the solder spot on the tabs mates with the gold on the connector an unnacceptably high contact resistance results.
Efforts that were tried to solve this problem of solder adhering to the tab protectors were:
a. To buff the protector surface. PA1 b. To coat it with polymerized silicon mold release. PA1 c. To coat it with teflon polyimide. PA1 d. To cover the protector with heat resistant tape. PA1 e. To coat it with flux. PA1 a. Mechanical wire brushing simply moved the bits around inside the protector, which because of their small size, had little weight and did not fall out or could not be washed out. PA1 b. Ultrasonic cleaning methods were unsuccessful. PA1 c. Chemical solder strippers were too agressive and attacked the Titanium.
None of these methods prevented the microscopic dots of solder from adhering to the tab protector, resulting in having to clean the bits of solder from the protector before re-use. This too was not satisfactory since:
Numerous handling methods were investigated to prevent the microscopic bits of solder from entering the protector but none were successful.
Since none of the above methods were fully successful, the use of the tab protectors alone was abandoned and the gold tabs were masked with a special heat resistant tape. This method prevented solder from contaminating the tabs but incurred additional labor costs to apply and remove the tape and the tape cost itself was excessive. The tab protector was still used over the tape for its stiffening qualities.